Compounds represented by the following formula (II) are known as heterocyclic compounds having cardiotonic activity and are disclosed for example in Japanese Patent Provisional Publication (Kokai) Nos. 57-2284 and 58-131981 and Japanese Patent Publication (Kokoku) No. 61-53350. ##STR4## wherein R.sub.1 and A-E are as defined above.
These known heterocyclic compounds having cardiotonic activities were clinically unfavorable since they have not so strong cardiotonic activities and increase heart rate.
We, the inventors carried out studies to succeed in synthesizing novel heterocyclic compounds having excellent positive inotropic effect as cardiotonic agent and suppressing increase of heart rate, by modifying the pyridyl group in the compound of the formula (II), thus completing the present invention. More specifically, the present invention is directed to tetrahydropyridine derivatives represented by the above-mentioned formula (I) or pharmaceutically acceptable acid addition salts thereof and cardiotonic agents containing them as effective components.